Hydraulic system



Nov. 23, 1965 F. c. PRUVOT HYDRAULIC SYSTEM 4 Sheets-Sheet 1 Filed April 2, 1964 IN VEN TOR. fizz/(013' v BY Ac-s i /arias. x

Nov. 23, 1965 F- c. PRUVOT 3,213,805

HYDRAULIC SYSTEM Filed April 2, 1964 4 Sheets-Sheet 3 INVENTOR- (f firm 0X,

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memauwaw 02/1 7616 mwzaz Pl/MP I N VEN TOR fizz/(v11 (I ry/02 United States Patent 3,218,805 HYDRAULIC SYSTEM Francois C. Pruvot, Paris, France, assignor, by mesne assignments, to TRW linc., a corporation of Ohio Fiied Apr. 2, 1964, Ser. No. 356,851 19 Claims. (Ci. 60-52) This invention relates to hydraulic servo or poweramplifying systems, and more particularly to that type of system in which the delivery of fluid from a highpressure source to a reversible hydraulic motor is controlled in response to a pressure diflerential created by operation of an input or control pump. Systems of this type are well adapted for use in power steering gears for vehicles, the control pump being operated by the vehicle steering wheel and the source of fluid under pressure being a pump conveniently driven from the vehicle engine.

Examples of systems of the general type referred to are found in US. Patent No. 2,995,012, granted August 8, 1961, on the application of E. W. Cassaday et al. In the systems there shown, the control pump is arranged in a fluid circuit with the reversible motor and a valve means which operates in response to the pressure differential created by the control pump to control the admission to the circuit of high-pressure fluid delivered from a power-operated pump. A characteristic of the systems specifically shown and described in Patent No. 2,995,012 is that all the fluid displaced in operation of the motor flows through the control pump. If, as is frequently desired in a system of the general type referred to, the design is such as to enable the motor to be operated by the control pump in event of failure of the power-driven pump, the characteristic just mentioned may have the consequence that in normal system operation the control pump, in order to create the pressure differential necessary to maintain the supply of high-pressure fluid to the motor, will have to be operated at an objectionably high rate of speed. To avoid such necessity for operating the control pump at an objectionable speed, it was proposed in US. Patent No. 2,974,491, issued March 14, 1961, on the application of E. W. Cassaday et al., that the valve means incorporated in the circuit be such as will open a by-pass around the control pump whenever high-pressure fluid is admitted to the circuit.

During power operation of the system disclosed in Patent 2,974,491, the ratio of the system-4e, the ratio of the rate of control-pump operation to the rate of motor displacement-depended on the ratio between the respective rates of fluid flow through the control pump and through the by-pass, and the patent contemplated that the latter ratio would be primarily controlled by the relative size of orifices located in the by-pass and in the control-pump circuit around the by-pass. It has been found in practice that while the system disclosed in Patent 2,974,491 does function to reduce the speed at which the control pump must be operated to maintain the supply of high-pressure fluid to the motor, the system-ratio it provides during power operation tends to vary more or less uncontrollably with the rate of fluid flow, especially when the flow rate is relatively low. In Patent 2,974,491, the orifices which effected the division of flowing fluid between the control pump and the bypass around it were disclosed as of fixed sizes; and the aforesaid variations in system-ratio are believed to be due, at least in large part, to the fact that flow through an orifice of fixed size becomes increasingly sensitive to variations in the pressuredrop across the orifice as that pressure-drop decreases.

It is therefore an object of this invention to provide a hydraulic servo or power-amplifying system in which, over a wide range of flow-rates including low flow-rates, the rate of flow of pressure liquid from a power-operated pump to a reversible motor will bear a definitely controlled relation, usually although not necessarily, linear, to the rate of flow through a control or input pump. A further object of the invention is to produce such a system in which, in event of failure of the power-operated pump, the motor can be operated by the control pump at a mechanical advantage independent of any relation which, when the power-operated pump is functioning, is maintained between the respective flow rates through the control pump and the motor.

A hydraulic system suitable for incorporation of my invention comprises a main, or output, circuit and a control, or input circuit. The main circuit comprises supply means including a power-driven pump for delivering fluid under pressure, a restricted orifice receiving fluid from the supply means, and a reversible hydraulic motor receiving fluid passing through the orifice. The control circuit embodies a control or input pump and a second restricted orifice through which passes liquid displaced in operation of the control pump. Conveniently, especially if the control pump is to be capable of operating the motor in event of failure of the power-driven pump, fluid flowing in the control circuit is drawn from and returned to the main circuit. Valve means responsive to the existence and sense of a pressure differential created by operation of the control pump controls the circuits in such a way that, although flow through the orifices will always be in the same direction, the direction of motor operation will depend upon the direction in which the control pump is operated.

In incorporating my invention in such a system, I employ means responsive to flow conditions in the control circuit for so regulating flow conditions in the main circuit that the respective rates of flow in the two circuits will always hear a controlled relation to each other. In the preferred arrangement, the two orifices are each of variable area, the main circuit includes means for regulating the pressure drop across the main-circuit orifice, and mechanism operative in response to the pressure drop across the control-circuit orifice jointly controls the respective areas of the orifices and also operates the aforesaid regulating means to provide across the main-circuit orifice a pressure drop coordinated with that across the control-circuit orifice.

The simpler forms of the invention as hereinafter described maintain a constant ratio between the respective areas of the orifices and equality of the pressure-drops across the orifices, with the result that the system-ratiohe, the ratio between the rate of control pump operation and the rate of motor response-will be the same at all flow rates. In its broader aspects, however, my invention is not limited to systems providing a constant system ratio, but rather contemplates systems in which that ratio is accurately determined whatever the flow rate may be. Accuracy in controlling the system ratio obviously requires accurate control of the relative flow rates in the two circuits; and, for reasons to be explained below, accuracy in the latter respect is greatly promoted by varying the orifice areas in the same sense as that in which flow rates vary.

Further objects and features of the invention will become apparent from the following more detailed description and from the accompanying drawings in which:

FIG. 1 is a schematic view of a complete servo system;

FIG. 2 illustrates schematically a portion of the system shown in FIG. 1 in association with a preferred form of orifice-controlling means shown in axial section in the condition existing when the system is at rest;

FIG. 3 is a sectional view of the orifice-controlling means of FIG. 2 illustrating a condition existing when the system is in operation;

FIG. 4 is a half-section showing the orifice-controlling 3 means of FIGS. 2 and 3 modified to provide a different arrangement of orifices;

FIG. 5 is a schematic illustration of a complete system embodying a modified form of means for regulating the pressure drop across the main-circuit orifice;

FIG. 6 is another schematic illustration of a complete system showing a further modification of the last mentioned means;

FIG. 7 is a simplified schematic view illustrating an operating principle;

FIG. 8 is a fragmental section similar to FIG. 3 showing a modified form of orifice-providing means;

FIG. 9 is a fragmental schematic view showing a modification of the systems illustrated in FIGS. 1, 5, and 6; and

FIG. 10 is a diagrammatic view, illustrating a vehicle steering gear embodying the invention.

A fundamental operating principle employed in the various complete forms of apparatus hereinafter described is illustrated in FIG. 7. That figure shows, greatly simplified, a hydraulic system operating in one direction. That system embodies a control circuit 10, in which fluid displaced by a control pump 11 passes through a variablearea orifice 12, and a main circuit 13, in which fiuid drawn from a sump 14 by a power-driven pump 15 passes through, in order, a pressure regulator and a second variable-area orifice 16 to a hydraulic motor 17. In addition to the two circuits, the system includes an operator that controls the variable-area orifices through means including mechanical connections diagrammatically indicated at 18. As shown, the operator comprises a spring 19, which biases the orifice-controlling means in the orifice-closing direction, and a hydraulic device 20, indicated as of the cylinder-and-piston type, which operates in opposition to the spring 19 to open the orifices in response to the pressuredrop across the orifice 12 in the control circuit. The pressure regulator operates, likewise in response to such pressure drop, to maintain a coordinated pressure drop across the orifice 16 in the main circuit.

As will be readily apparent, with the control pump 11 operating to cause fiuid in the control circuit to flow in the direction indicated by the associated arrow, the pressure drop across the orifice 12 will be applied to the cylinder of the device 20 and the operator will function, against effort of the spring 19, to open the orifice 12 to the extent necessary to establish a steady-state condition in which the eifort of the spring is balanced by the effect of the pressure drop on the device 20. At the same time the operator functions to open the orifice 12 it also functions to open the orifice 16 to a related extent. The pressure regulator, under control of the pressure drop across orifice 12, functions to maintain a related pressure drop across orifice 16. Since the rate of flow in the main circuit 13 is determined jointly by the area of and pressure drop across orifice 16, and since that area and pressure drop, through functioning of the operator and pressure regulator, will always be coordinated respectively with the area of and pressure drop across the orifice 12, the rate of flow in the main circuit 13 should always hear a controlled relation to that in the control circuit 10. Theoretically, that result would be attained even if the areas of the orifices were fixed; but, since the pressure drop across an orifice varies as the square of the flowrate, friction and other uncontrollable conditions may seriously interfere with accurate control of flow-rates by pressure regulation alone.- This is especially the case at relatively low flow-rates; for if a fixed orifice is a large enough to prevent an objectionably large pressure drop at the maximum required flow-rate, then minute variations in the pressure drop at low flow rates can result in relatively large variations in flow. Where the pressure drop across a main-circuit orifice is to be responsive to that across an orifice in a control circuit and control of the system as a whole is exercised by regulation of the flow-rate in the control circuit, the consequences of employing fixed-area orifices would be compounded; That is, at low operating rates the pressure drop across the control-circuit orifice would be relatively insensitive to variations in the rate of flow through that orifice, while the flow-rate through the main-circuit orifice would be relatively highly sensitive to the pressure drop there. It is for this reason that I prefer to employ variable-area orifices and to vary their area in the same sense as flow-rates vary.

As above noted, FIG. 7 is presented merely to illustrate a principle of operation, and it is to be understood that a system employing a reversible motor and so arranged that the motor can be operated by the control pump in event the power-driven pump fails requires components in addition to those shown in that figure. A system embodying such additional components is shown in FIG. 1.

In a system according to FIG. 1, the control pump 11 is connected to the orifice 12 and the motor 17 to the orifice 16 through a control valve 25 of the double-inverter type. To that end, the valve 25 is indicated as connected to eight conduits or fiuid passages 2633. Conduits 26 and 27 are control conduits extending to the control pump 11, while conduits 28 and 29 are motor conduits extending to opposite ends of a cylinder 34 which, with its associated piston 35, form part of the motor 17. Conduit 30, receives fluid under pressure from the -power-driven pump 15 and contains the orifice 16 and a check valve 36 located between such orifice and the valve 25. Conduit 31 is a return conduit extending to the sump 14 from which the pump 15 draws. Conduit 32 extends to one side of the orifice 12, while the other side of such orifice is connected through a conduit 37 to the supply conduit 30 at a point between the check valve 36 and the control valve 25. Conduit 33 is connected to supply 30 between check valve 36 and orifice 16.

To correlate FIGS. 1 and 7, it may be noted that in FIG. 1 the conduits 33, 27, 26, 32 and 37 form parts of a control circuit including the control pump 11 and orifice 12 and corresponding to the circuit 10 of FIG. 7; that conduits 30, 28, 29 and 31 form parts of a main circuit containing the power-driven pump 15, orifice 16, and motor 17 and corresponding to the circuit 13 of FIG. 7; and that the stretch of conduit 30 containing the check valve 36 is common to both circuits. FIG. 1, in agreement with FIG. 7, indicates schematically that the orifices 12 and 16 are adjustable by means including the operative connections 18 extending to the operator 1920. In order that such operator may be responsive to the pressure drop across the orifice 12, opposite ends of the cylinder 20 are connected to the conduits 32 and 37 through lines 38 and 39. The pressure regulator, which controls the pressure drop across the main-circuit orifice, is shown in FIG. 1, but will not be described until later.

The manner in which the control pump 11 is connected in the supply circuit and the motor 17 in the main circuit is determined by the condition of the control valve 25, which, in so functioning, operates to control the connections of control conduits 26 and 27 on the one hand to conduits 32 and 33 on the other hand and also to control the connections of motor conduits 28 and 29 on the one hand to conduits 30 and 31 on the other hand. As shown, the valve 25 is of the closed-center type having a neutral or center position in which it blocks communication of control conduits 26 and 27 with conduits 32 and 33 and communication of motor conduits 28 andv 29 with conduits 30 and 31. Associated with the valve 25 is a mechanism, described below, for shifting the valve in one direction or the other from its central position in response to the differential pressure resulting from operation of the control pump 11. Shifted to the right, the valve will establish the connections indicated in rectangle A, conduits 26, and 27 becoming connected respectively to conduits 32 and 33, while conduits 28 and 29 respectively become connected to conduits 30 and 31. Shifted to the left, the valve establishes the inverted connections indicated'in rectangle B, conduit 26 becoming connected to conduit 33, conduit 27 to conduit 32. conduit 28 to conduit 31, and conduit 29 to conduit 30. The means employed to control the valve 25 is shown as of a not uncommon type comprising a pair of oppositely acting springs 40, which may be preloaded, acting to bias the valve toward its neutral position, and a pair of hydraulic devices 41 and 42 which also act oppositely to each other on the valve to produce a net valveshifting effort responsive to the pressure differential created by the control pump 11. To that end, the two devices 41 and 42 are connected through lines 43 and 44 respectively to the control conduits 26 and 27.

The normal operation of the system as so far described is as follows: With no input effort applied to the control pump 11, pressures in the conduits 26 and 27 and in the two valve-controlling devices 41 and 42 will be equal and the valve 25 will be in its neutral position preventing fluid flow to and from the control pump and motor and isolating the control pump from any load on the motor. If the control pump is operated in a direction to move fluid from conduit 27 to conduit 26, pressure in conduit 26 and in device 41 will rise, while pressure in conduit 27 and in device 42 will drop, causing the valve to shift to the right to establish the connections indicated in rectangle A, thus applying to the operator-cylinder the pressure differential created by the control pump and causing the operator to open the orifices 12 and 16. As a result, fluid withdrawn from the sump by the poweroperated pump 15 and delivered under pressure to conduit 30 will flow through orifice 16 and immediately therebeyond will divide into two streams, one passing directly to the valve through check valve 36 and the other passing through the control circuit comprising conduits 33 and 27, control pump 11, conduits 26 and 32, orifice 12, and conduit 37 to conduit 30. The rejoined streams entering valve 25 will be directed thereby into conduit 28 for flow to the motor causing piston 35 thereof to shift to the right, while fluid displaced by rightward movement of the motor piston will return to the sump 14 via conduit 29, valve 25 and conduit 31.

If the control pump 11 is operated in the reverse direction, reverse operation of the motor 17 will result, as in that case, the valve 25 will be shifted to the left to establish the connections indicated in rectangule B, and the fluid delivered to the valve through conduit will be directed into motor conduit 29 rather than into conduit 28, motor piston will be moved to the left, and fluid displaced from the motor will return to the sump through conduits 28 and 31. In the reverse operation of the control pump, fluid will still be drawn from conduit 30 via conduit 33 and returned via orifice 12 and conduit 37, since the leftward shift of the valve connected conduit 33 to control conduit 26 and control conduit 27 to conduit 32.

In either direction of operation of the control pump, operation of the system will continue only so long as the control pump is being operated at a rate such that the pressure diiferential it creates is sutficient to maintain the valve 25 shifted from its neutral position. When operation of the control pump is terminated, that pressure difierential disappears, the valve 25 assumes its neutral position, the spring 19 closes the orifices '12 and 16, and the motor 17 comes to rest.

If, as is desirable, the check valve 36, which has no function in normal operation of the system, is not loaded, the pressures on the downstream sides of the orifices 12 and 16 will be the same, and the rates of flow through the orifices will depend upon their areas and the pressures on their upstream sides. I contemplate that those upstream pressures will be coordinated and, in the simplest case, that they be equal so that, if the discharge coefficients of the orifices are equal, the respective flow rates, the relation between which determines the system-ratio, will depend substantially solely on the relation between the orifice areas.

Various means may be employed for maintaining coordinated upstream pressures at the orifices 12 and 16. In the arrangement shown diagrammatically in FIG. 1, which will be described as designed to maintain those upstream pressures equal, a by-pass containing an adjustable valve 51 is provided between the inlet of conduit 30 and the sump 14. Mechanism including hydraulic devices 52 and 53 acting oppositely on the valve 51 and jointly responsive to the pressure-drops across the orifices 12 and 16 automatically adjusts the valves 51 to maintain those pressure drops equal other. To permit the valve 51 to be of a simple type in which Bernouilli efifects are not balanced, I prefer to operate it through the medium of a pilot valve 54 which, in response, jointly to the pressures in conduits 30 and 32, controls the admission and exhaust of fluid to and from the devices 52 and 53, such fluid conveniently being supplied by the pump 15. As shown, the pilot valve is a simple closed-center inverter valve controlling communication of two conduits 55 and 56 extending from the devices 52 and 53 with -a conduit 57 leading to sump 14 and a conduit 58 extending to the outlet of pump 15. The pilot valve is biased toward its center position as by opposed springs 59 and is adapted to be shifted from such position by the net effect of two oppositely acting hydraulic devices 60 and 61 respectively connected through lines 62 and 63 to the conduit 30 and the line 38 to be responsive to the respective pressures on the upstream sides of the orifices 16 and 12.

The pressure controlling apparatus just described operates as follows: The pressure upstream of orifice 16 will be equal to the pressure at the inlet of bypass 50, which latter pressure will vary with the pressure-drop across the valve 51. If the pressure in conduits 30 and 32 are equal, the pressures in the operating devices 60 and 61 of pilot valve 54 will also be equal and such valve will occupy its neutral position preventing any flow in conduits 55 and 56, thereby maintaining the bypass valve 51 in the condition which causes the pressure in conduit 30 to equal that in conduit 32. An increase in pressure at the upstream side of orifice 12 will be transmitted through line 63 to valve-operating device 61, which will function to shift the pilot valve 54 to connect conduit 55 to conduit 58 and conduit 56 to conduit 57. The resultant relief of pressure in the device 53 of valve 51 and the imposition of pump pressure on the oppositely acting device 52 will shift bypass valve 51 toward closed position to increase pressure in conduit 30 at the upstream side of orifice 16. When the latter pressure is brought to that at the upstream side of orifice 12, the pressures in the operating devices 60 and 61 will become equal and the valve 54 will assume its center position to maintain the bypass valve 51 in its new position. A drop in pressure at the upstream side of orifice 12 will have the opposite effect, the resultant drop in pressure in operating device 61 shifting the valve 54 to create in the devices 52 and 53 a differential pressure causing the valve 51 to move toward open position and reduce pressure in conduit 30. To insure in the line 58 the existence of pressure adequate to operate the bypass valve 51, the connection of the pump 15 to the conduit 30 desirably includes a springloaded valve 67 which can operate, by excluding or reducing flow from the pump 15 to conduit 30, to provide for the pump a predetermined minimum discharge pressure for application to the line 58.

The bypass valve 51 functions as a pressure-relief valve for the pump 15. Thus, when the valve 25 is in its neutral posit-ion blocking flow through supply conduit 30, the resultant high pressure in that conduit, applied to the device 60 of pilot valve 54 will operate that valve to cause opening of the bypass valve 51 and permit the fluid discharged by pump 15 to flow to the sump.

The apparatus shown diagrammatically in FIG. 1 is adaptable for operation by the control pump 11 alone in event of failure of the power-operated pump 15. For

that purpose, the conduit 33 is connected to the sump 14 through a conduit 65, which includes a check valve 66 opening toward the valve 25. So long as the pump 15 is functioning, pressure in conduit 30 maintains the check valve 66 closed and liquid displaced in operation of the control pump 11 is drawn, through conduit 33, from conduit 30 on the upstream side of the check valve 36 and returned, through conduits 32 and 37, to conduit 30 on the downstream side of such check valve; but if the pump 15 fails, liquid displaced by operation of the control pump can be drawn from the sump 14 past check valve 66 and will be delivered through conduits 32, 37, 3t), and valve to one or the other of the motor conduits 28 and 29 to effect operation of the motor 17. In such operation, check valve 36 prevents escape of fluid put under pressure by pump 11.

In considering the schematic showing of FIG. 1, as well as certain other figures in the drawings, it is to be borne in mind that at least in some instances elements referred to as conduits or lines need not be separate elements such as pipes, tubes, or hoses but may be simple passages formed in other elements of the system. For example, it may be convenient to incorporate some or all of the various valves, and even the orifices 12 and 16 and their controlling mechanism, in a single housing provided with passages serving the purpose of elements sometimes designated herein as conduits or lines.

In FIGS. 2 and 3, I have shown somewhat diagrammatically and indicated by the reference numeral 70 a suitable form of means for providing and controlling the orifices 12 and 16, such means being inserted between the conduits 32 and 37 and in the conduit on the up stream side of the connection of that conduit to the conduit 33. For convenience in explaining the construction shown in FIG. 2, that section of conduit 30 through which fluid leaves the means is designated 30a while that section which conducts pressure fluid to the means 70 is designated 30b.

The means 70 is shown in FIGS. 2 and 3 as comprising a housing 71 provided with a deep cylindrical recess 72 and also with ports 73, 74, 75, and 76 for connection respectively to the conduits 30a, 32, 37, and 30b. The outer end of the recess 72 is closed by a stationary plug 78 between which and the base of the recess a piston 79 is slidable. The plug 78 has an axial bore slidably receiving a spool 80 having a thrust-transmitting connection with the piston 79. As shown, such connection is adjustable, comprising a pin 81 which projects from the inner end of the spool 80 for engagement with the piston 79 and which, at its opposite end, has a head screw-threadedly received in a recess in the outer end of the spool. spring 19 of FIGS. 1 and 7, which is shown in FIGS. 2 and 3 as housed in a cap 82 secured to. plug 7 8, acts against the outer end of spool 80 to urge it and the piston 79 toward the base of the recess-72.

In the specific means 70 shown in FIGS. 2 and 3, the.

equivalent of each of the orifices 12 and 16 is, for a reason and in a manner which will become apparent hereinafter, dual in character, being provided by two annular openings which are connected in series in the case of the equivalent for orifice 12 and in parallel in the other case. The two openings constituting the equivalent for orifice 12 are defined by cooperating provisions on the plug 78 and spool 80, while those constituting the equivalent for orifice 16 are defined by cooperating provisions on the piston 79 and housing 71.

To provide the former pair of openings above mentioned, the plug 68 is formed interiorly with three axially spaced annular grooves 83, 84, and 85 and the spool 80 with two axially spaced exterior annular grooves 86 and 87. To provide the latter pair of openings, the piston 7 is formed with two axially spaced exterior grooves 89 and 90 cooperating with four axially spaced annular grooves 91, 92, 93, and 94 provided in the cylindrical wall of recess 72 in the housing. .The plug-groove 83 communicates with housing port 74 and, through a passage The 96, with the base of recess 72, while the plug-groove 85 communicates with housing port and, through a passage 97, with both the interior of cap 82 and the space between the plug 78 and piston 79. The housing grooves 91 and 93 both communicate with housing inlet port 76, while the grooves 92 and 94 both communicate with housing outlet port 73.

When the control pump is not in operation and no fluid is being displaced in the system, the pressures at the housing ports 74 and 75 will be equal and, transmitted through passages 96 and 97 to opposite ends of the piston 79, will counteract each other, permitting the spring 19 to hold the spool and piston 79 at the limit of their movement to the right toward the base of recess 72. In that condition, hereinafter referred to as the neutral condition, the spool grooves 86 and 87 respectively lap the plug grooves 83 and 84 but the groove 86 lies Wholly to the right of plug groove 84 and the groove 87 lies wholly to the right of plug groove 85, thus blocking communication between housing-ports 74 and 75. Similarly, piston grooves 89 and 90 respectively lap housing-grooves 91 and 93, but the grooves 89 and 90 lie wholly to the right respectively of housing grooves 92 and 94, thus blocking communication between the housing ports 76 and 73. As noted below, complete blocking of communication between the ports 76 and 73 in the neutral position of piston 79 is not essential. Both the spool grooves 86 and 87 and the piston grooves 89 and 90 are wide enough that, throughout the entire range of movement of the spool and piston, each will remain in open communication with the plug groove or housing groove which it laps in the neutral position.

When the control pump 11 is operated to cause shifting of the valve 25 and creation of a pressure differential between the conduits 32 and 37 as previously described, the pressure at housing port 74, transmitted through passage 96 to the right hand end of the piston, will rise while the pressure at housing port 75, transmitted through passage 97 to the left hand end of the piston, will drop to create a resultant force urging the piston and the spool to the left against the force exerted by the spring 19. Leftward movement of the piston 79 and spool 80 as a result of the pressure differential applied to housing ports 74 and 75 creates the condition shown in FIG. 3. In that condition, spool grooves 86 and 87 communicate respectively with plug grooves 84 and through annular orifice openings 12a and 12b, while piston grooves 89 and communicate respectively with housing grooves 92 and 94 through annular orifice openings 16a and 16b. As previously mentioned the orifices 12a and 12b are series-connected, While the orifices 16a and 16b are parallel-connected.

The modified form of orifice-providing means shown in FIG. 4 embodies a spool 80', which is the same as the spool 80 except that the land between the grooves 86 and 87 is eliminated, and a piston 79' which is the same as the piston 79 except that the groove 90 is eliminated. Elimination of the land from spool 80 leaves the plug groove 84 permanently in open communication with the plug groove 83-, thus eliminating the variable-area orifice opening 12a while retaining provision for the other opening 12b. Similarly, elimination of the piston groove 90 results in permanent occlusion of the housing grooves 93 and 94, leaving the single variable-area orifice opening 16a in the flow path between housing ports 76- and 73.

Provision of alternative spools 80 and 80 and of alternative pistons 79 and 79 makes it possible, without changing the construction of the housing 71 or plug 78, to obtain incremental variation of the system-ratio over a rather wide range. For any given pressure drop between. the ports 76 and 73 in any given piston position, the flow rate provided by the single orifice 16a of FIG. 4 will be half that provided by the parallel-connected orifices 16a and 16b of FIG. 3. In the case of the series-connected openings 12a and 12b of FIG. 3, the pressure-drop across 9 housing ports 74 and 75 will be divided equally between those openings and the flow-rate for any given over-all pressure-drop will be about 71% of that provided by the single opening 12b in the construction of FIG. 4. If the pressures at housing inlet ports 74 and 76 are equal and if the diameter of the piston is, say five times that of the spool, various combinations of pistons 79 and 79' with spools 80 and 80 will vary the ratio of the flow rate between housing ports 76 and 73 to the flow rate between housing ports 74 and 75 as follows:

As will be obvious, a wider range of flow-rate ratios, or smaller incremental variations thereof, can be obtained by increasing the number of plug and housing grooves and the number of alternatively useable pistons and spools.

The above table showing the effect of using spool and piston constructions which provide, alternatively, either multiple or single orifice openings presupposes that the spool-controlled openings will have equal diameters, that the piston-controlled openings will have equal diameters, and that all openings will be established at the same point in leftward movement of the spool and piston from their neutral condition shown in FIG. 2. For simplicity of design and construction, it is desirable that the spool-controlled openings have the same diameter and be established at the same point in leftward movement of the spool; and, for the same reasons, it is desirable that the piston-controlled openings have equal diameters and be established at the same point in leftward movement of the piston. As brought out below, it is not essential that the spool-controlled and piston-controlled openings be established at the same point in leftward movement of the spool and piston or that those openings have the same width. Such factors can be controlled by adjustment of the pin 81 in the spool 80.

The spring 19, which opposes opening of the variablearea orifices should be preloaded at least to the extent necessary to insure that the piston 79 and spool 80 will be at the rightward limit of their movement when no liquid is being displaced in the system. Preferably the spring is one having a relatively low rate in order to reduce the extent of variation in the force with which it opposes leftward movement of the piston 79 and spool 80. The preloading of the spring 19 should be coordinated with that of the springs 40 which bias the control valve 25 toward neutral position, in order that any differential pressure greater than that required to shift the valve 25 will be suficient to shift the piston 79 and spool 80 of the orificeproviding means 70.

If the pump 15 is a constant-displacement pump, the mechanism shown schematically in FIG. 1 for controlling the pressure in conduit 30 upstream of the orifice 16 is not well suited for use in situations where that pump supplies liquid to apparatus other than the system constituting the subject matter of this application, as the demands of such other apparatus might unduly lower the pump-outlet pressure. An alternative pressure controlling mechanism suited for use in situations where a common poweroperated pump supplies both my system and other equipment is embodied in the system of FIG. 5. That system is the same as the one shown in FIG. 1 except for the mechanism employed to control pressure at the upstream side of the orifice 16. In it, the pump 15 of FIG. 1, which can be a constant-displacement pump, is replaced with a pressure compensated variable-displacement pump 100 which embodies mechanism responsive to the outlet pressure of the pump for varying pump-displacement in a sense opposite to that in which such outlet pressure varies. In that manner, should equipment supplied as through a conduit 101 create a demand tending to lower pumpoutlet pressure, the pump displacement would be increased to restore the outlet pressure. In this instance, the pump-outlet is connected to the conduit 30 through a simple two-way valve 102 similar to the valve 51 and operating in response to the difference in the respective pressures in the devices 52 and 53, but differing from the valve 51 in that its response to such pressure diflerence is opposite that of the valve 51. That is, the valve 102 will be shifted toward open position to increase pressure in conduit 30 when the pressure in the device 52 exceeds that in the device 53 and toward closed position to lower pressure in conduit 30 when the pressure in device 53 exceeds that in device 52.

The over-all operation of the system shown in FIG. 5 is essentially the same as that of the system shown in FIG. 1. When the control pump 11 is not being operated, the valve 25 is in neutral position and there is no flow of liquid to the motor 17. Upon operation of the control pump, the control valve 25 will be shifted in one direction or the other to permit liquid discharged by the power-operated pump to flow to the appropriate end of motor 17 and liquid displaced by operation of the control pump 11 to flow through orifice 12 to the motor. As before, the operator 19-20 operates to control the areas of the orifices 12 and 16 in response to changes in the pressuredrop across the orifice 12, and the pilot valve 54 controls the valve 102, as above described, to maintain the pressure at the upstream side of orifice 16 equal to that at the upstream side of orifice 12.

When the sole function of the power-operated pump is to provide liquid for my system, the pressure-controlling mechanism can be considerably simplified. One such simplified arrangement, illustrated in FIG. 6, eliminates the valve 51 (102) and its controlling pilot valve 54 and employs a so called servo pump, or a variable displacement pump embodying a hydraulic displacement-controlling device 106, responsive to the difference between the pressures at the upstream sides of the orifices 12 and 16. As will readily be appreciated, the device 106 operates to increase pump displacement, and hence to increase pressure of the liquid delivered to conduit 30, when the pressure at the upstream side of orifice 12 increases and to decrease the pressure of liquid supplied to conduit 30 when the upstream pressure at orifice 12 decreases. The over-all operation of the system of FIG. 6 is the same as that of the systems of FIGS. 1 and 5.

As so far described, the systems of FIGS. 1, 5, and 6 0perate to maintain a linear relation between the respective flow-rates in the control and main circuits, and do so by maintaining a linear relationship between the areas of the two orifices and equality between the respective pressure drops across them. Should a non-linear relation between the two flow-rates be desired, that result could be attained by modifying the relation maintained either between the orifice areas or between the respective pressure drops.

In the discussion of FIGS. 3 and 4 above, I have indicated how the ratio between the flow-rates in the main and control circuits can be varied without altering the diameters of spool-type orifice-providing means in which the two spools shift as a unit. Another manner in which such ratio can be varied without changing spool diameters is by interconnecting the spools through a means so constructed that the spools move from and toward their respective neutral positions at different rates. One such arrangement, shown in FIG. 8, interconnects the piston 79 and the spool 80 through a lever in the form of a disk 110 retained by a ring 111 in an axial recess in the inner end of the plug 78 and engaged on one side by the pin 81 and on the other side by a pin 112 mounted eccentrically in the piston 79. In transmitting thrust between the piston and spool, the disk 110 fulcrums in the ring 111 at a point diametrically opposite the pin 112, with the result that the main-circuit orifice opening or openings defined by the piston will open and close more rapidly than the control- 1 1 circuit orifices defined by the spool, and the system ratio will be correspondingly increased.

In FIG. 9 I have shown an arrangement by which any of the systems heretofore described can be adapted for operation alternatively as a servo system or as a system in which the follow-up feature characteristic of a servo system is absent. That arrangement is the same as any of the others shown except for the provision of a valve 115 connected in series with the control-circuit orifice 12 between the conduits 32 and 37. With that valve open, the system operates as before, but if the valve is closed no flow in the control circuit is possible and effort applied to the control pump 11 can do no more than create a differential pressure which will shift the control valve 25 and open the orifices. Opening of the orifice 12 would of course have no effect, because of the closed condition of the valve 115, but opening of the orifice 16 would permit pressure fluid to flow to the control-valve which, in its shifted condition, would direct such fluid to the proper end of the motor 17. The rate of flow to the motor will depend on the extent to which the orifice 16 is opened; and if opening of the orifices is opposed by a spring 19, the extent to which the orifices open will depend upon the effort applied to the control pump. The sensitivity of motor response to that effort can be decreased and control of the motor accordingly improved by employing a spring 19 of relatively high rate. However, as above noted, increasing the spring rate promotes non-linearity of the relation between flow-rate and orifice-opening.

FIG. illustrates diagrammatically a power steering gear embodying my invention. As there indicated, the control pump 11 is a gear pump operated by the steering wheel 120 of the vehicle, and the motor 17 operates the conventional pitman 121 adapted to be connected to the steered wheel or wheels of the vehicle.

In all the systems above described, the orifice-closing effort is exerted by a spring 19; and since such a spring obeys Hookes law, the orifice-closing effort and the pressure drop necessary to counteract it will vary with the area of the orifice 12. It is not necessary, however, that such orifice-closing effort and pressure drop vary with the orifice area, as the systems would work just as well if the spring 19 were replaced by a means which exerted a constant orifice-closing effort throughout the entire range of orifice adjustment. This is so because any change in the pressure drop across the orifice 12 changes the area of that orifice in a sense which tends to restore the pressure drop to its former value. The less the orificeclosing effort varies with changes in orifice area the less will the relation between orifice area and flow-rate depart from linearity. The extent of variations in an orificeclosing effort exerted by a spring, such as the spring 19, will obviously depend upon the rate of the spring, and it is entirely practicable in the practice of my invention to employ a spring 19 of such a low rate that the departure from linearity of the relation between orifice area and flow-rate will be insignificant. Accordingly, although readjustments of the orifice-providing means are occasioned directly by changes in the pressure drop across the orifice 12, it is not inaccurate to refer to that means as providing for the main-circuit orifice 16 an area proportioned or responsive to the flow-rate in the control circuit.

In addition to promoting linearity in the relation of orifice area to flow-rate, the use of a spring 19 of low rate lessens variations in the effort which must be applied to the control pump 11 in order to create the pressure differential necessary to hold the orifices open against the effort exerted by the spring. In most instances, it probably will be desirable to keep as nearly constant as possible the effort which must be applied to the control pump to open the orifices. However, in a system such as that shown in FIG. 9, there can be an advantage in using a spring of a rate high enough to result in substantial variations in the effort applied to the control pump.

This is perhaps most readily made apparent by considering the system of FIG. 9 embodied in a steering gear such as shown in FIG. 10. With the valve of such a system closed, the rate at which the steered wheels are displaced will depend upon the displacement of the steering wheel from a neutral position. As the range of steering wheel movement with the valve 115 closed is limited, the response of the motor to steering wheel displacement might be objectionably sensitive unless movement of the steering wheel from the neutral position is opposed by an ever-increasing effort such as would be supplied by a spring 19 of relatively high rate.

In all the systems so far described, the pressure-regulating means of the main circuit is responsive to the pressure-drop across the orifice 12 in the control circuit. This is not necessary, as the flow rate in the main circuit can be coordinated in any desired manner with that in' the control circuit even if the pressure regulating means operates independently of the pressure-drop across the control-circiut orifice. For example, if, in accordance with the possibility mentioned above, the yieldable orificeclosing means is one which exerts a constant effort irrespective of the extent of orifice opening, the pressure-drop across the control-circuit orifice 12 will always be constant and known, and if the relation between orifice-areas is linear a constant system ratio could be provided merely by arranging the pressure regulating means of the main circuit to maintain the same known pressure-drop across the main-circuit orifice 16. If the main-circuit pressureregulating means is one which maintains constant the pressure-drop across the main-circuit orifice 16, the systemratio will be affected by the characteristics of the yieldable orifice-closing means, which can be so designed that the orifice-closing effort it exerts varies in any desired manner with changes in flow rate. In short, so long as the variable area of the control-circuit orifice and the pressure-drop across it are automatically coordinated respectively with the variable area of the main-circuit orifice and the pressure-drop across it, the broad objects of my invention are obtainable.

Precise control of the ratio between flow rates in the control and main circuits can be interfered with by manufacturing inaccuracies and leakage in the controlcircuit. As will be obvious, leakage in the pump 11 or other elements of the control-circuit will reduce flow through the control orifice 12, thereby reducing the area of the main-circuit orifice 16 and the fiow rate in the main-circuit. The result on system ratio may be insignificant at medium and high flow rates, but may be objectionable at low flow rates. Leakage in the controlcircuit can be at least partially compensated for by so controlling the orifice-providing means that, in its operation, the main-circuit orifice 16 will be slightly open when the control-circuit orifice opens. In the orifice-providing means shown in FIGS. 2-4, the order in which the control-circuit and main-circuit orifices open is determined by adjustment of the pin 81 in the valve 80. Assume, for example, that any leftward movement of the piston 79 from its neutral position opens the orifice 16 but that the pin 81 is retracted into the valve 80 to such an extent that a finite leftward movement of that valve from its neutral position will be required before the orifice 12 opens. In that assumed situation, until the orifice 12 opens, both the pressure differential between its opposite sides and the flow rate in the main-circuit will be higher than they otherwise would be, and the affect of controlcircuit leakage on the system ratio will be reduced. If the lag in opening of the orifice 12 is properly fixed, the compensation for control-circuit leakage may be such that at intermediate and high flow rates the actual system ratio will have substantially its theoretical valve in spite of the leakage. It is possible that at and immediately adjacent the position occupied by the valve 80 when the orifice 12 just begins to open the lag in the opening of that orifice may result in some over-compen- 13' sation for leakage; but unless the lag is excessive, the result of that over-compensation would hardly be noticeable. It is to be understood that the lag necessary to compensate for any tolerable leakage in the control circuit will be but a small fraction of the total range of adjustment of the orifice-providing means.

As brought out above, it is not essential in any of the illustrated systems that the orifice 16 be completely closed when the orifice 12 is closed; for, as long as the control pump is at rest, no pressure-ditferenial will exist at opposite sides of the closed orifice 12, the pressure-regulating means in the main-circuit will insure the absence of a pressure-differential across the orifice 16, and there will be no flow in the main-circuit even if the orifice 16 is open. If the orifice 16 is open in the neutral condition, or if it opens before the orifice 12 opens, the system will operate at low rates of flow in somewhat the same manner as that in which the system of FIG. 9 operates at all flow rates when the valve 115 is closed to prevent flow through the control-circuit orifice. That is, until operation of the control pump creates a pressure-differential sufl'icient to open the orifice 12, the pressure-difierential it does create will, by its efiect on the pressure-regulating means, control flow in the main-circuit and the flow-rate in the maincircuit will vary with the etfort applied to the control pump.

It will be understood that the various apparatus and systems shown in the drawings and above described are set forth merely by Way of example anad may be varied without departing from the invention.

I claim as my invention:

1. A hydraulic system, comprising a control fluid circuit including a control pump, a main-circuit including a power-operated pump and a hydraulic motor, a first adjustable orifice-forming means associated with said control circuit for providing an orifice through which flows fluid displaced by said control pump, a second adjustable orifice-forming means associated with said main circuit providing an orifice through which flows fluid displaced in operation of said motor, mechanism responsive to the rate of fluid flow in said control circuit for controlling both said orifice-forming means to vary the areas of the orifices in the same sense as such flow-rate varies, and pressure-regulating means in said main circuit for automatically regulating the pressure drop across the orifice in that circuit.

2. A hydraulic system as set forth in claim 1 characterized in that said orifice-forming means and the mechanism controlling them are so constructed and arranged as to maintain a substantialy constant ratio between the areas of the respective orifices.

3. A hydraulic system as set forth in claim 1 with the addition of mechanism responsive to the pressure-drop across the control-circuit orifice for controlling said pressure-regulating means.

4. A hydraulic system as set forth in claim 3 characterized in that said pressure-regulating means and the mechanism controlling it are so constructed and arranged as to maintain the pressure-drop across the main-circuit orifice substantially equal to that across the control-circuit orifice.

5. A hydraulic system as set forth in claim 1 characterized in that said orifice-forming means and the mechanism controlling them are so constructed and arranged that the main-circuit orifice is always substantially larger than the control-circuit orifice.

6. A hydraulic system as set forth in claim 1 characterized in that in the main-circuit fluid flows from the power-operated pump through the main-circuit to the motor, a portion of the main-circuit between the maincircuit orifice and the motor being included in the control-circuit.

7. A hydraulic system as set forth in claim 1 characterized in that said mechanism is pressure-operated and 14 responsive to the pressure-drop across the control-circuit orifice.

8. In a hydraulic servo system having a reversible hydraulic motor, a reversible control pump, a poweroperated pump, a control valve controlling the connection of said pumps to said motor, and means responsive to the pressure-diflerential created by operation of the control pump for operating said control valve; means providing a first restricted orifice through which passes liquid flowing from the control pump to the motor, means providing a second orifice through which passes liquid flowing from the power-operated pump to the motor, both said orificeproviding means being adjustable to vary the area of the respective orifices, mechanism responsive to the rate of fluid flow through said first orifice for controlling the orifice-providing means to vary the areas of the orifices in the same sense as such flow-rate varies, and pressureregulating mechanism for automatically regulating the pressure-drop across said second orifice.

9. A hydraulic servo system as set forth in claim 8 with the addition of mechanism responsive to the pressuredrop across said first orifice for controlling said pressureregulating means.

10. In a hydraulic servo system having a reversible hydraulic motor, a reversible control pump, a poweroperated pump, and valve means operative in response to the existence and sense of a pressure-diflerential created by operation of the control pump for controlling the connection of said pumps to said motor, means providing a first restricted orifice through which passes liquid flowing from the control pump to the motor, means providing a second orifice through which passes liquid flowing from the power-operated pump to the motor, both said orificeproviding means being adjustable to vary the area of the respective orifices, mechanism responsive to the rate of fluid flow through first orifice for controlling the orificeproviding means to vary the areas of the orifices in the same sense as such flow-rate varies, and pressure regulating mechanism for automatically regulating the pressure-drop across said second orifice.

11. A hydraulic servo system as set forth in claim 10 with the addition of mechanism responsive to the pressure-drop across said first orifice for controlling said pressure-regulating means.

12. A hydraulic system, comprising a control fluid circuit including a control pump, a main-circuit including a power-operated pump and a hydraulic motor, a first adjustable orifice-forming means associated with said control-circuit for providing an orifice through which flows fluid displaced by said control pump, a second adjustable orifice-forming means associated with said main-circuit providing an orifice through which flows fluid displaced in operation of said motor, a normally open valve in said control-circuit, mechanism responsive to the pressuredrop across said valve and control-circuit orifice for controlling both said orifice-forming means to vary the areas of the orifices in the same sense as such pressure drop varies, and automatic means for controlling the pressuredrop across the main-circuit orifice, said valve, when closed, blocking fluid flow in the control-circuit while leaving said mechanism responsive to the pressure-diflerential created by eflort applied to the control pump.

13. A hydraulic system, comprising a control fluid circuit including a control pump, a main-circuit including a power-operated pump and a hydraulic motor, a first adjustable orifice-forming means associated with said control-circuit for providing an orifice through which flows fluid displaced by said control pump, a second adjustable orifice-forming means associated with said main-circuit providing an orifice through which flows fluid displaced in operation of said motor, mechanism responsive to the pressure-diflerential at opposite sides of the orifice-forming means in the control-circuit for controlling both said orifice-forming means to vary the areas of the orifices in the same sense as such pressure-differential varies, pressure-regulating means in said main-circuit for regulating the pressure-drop across the orifice in that circuit, and mechanism responsive to said pressure-ditferential for controlling said pressure-regulating means, said first named mechanism so controlling said orifice-forming means that the main-circuit orifice is open prior to opening of the control-circuit orifice.

14. A hydraulic system as set forth in claim 13 with the addition of adjustable means for varying the extent to which the main-circuit orifice is open when the control-circuit orifice opens.

15. In a hydraulic system, a first spool-type valve comprising a housing and a valve spool reciprocable therein, a control pump, means providing a control-circuit including said valve and control pump, a second spool-type valve comprising a housing and a valve spool reciprocable therein, said last named valve spool being of larger diameter than the spool of the first valve, a power-operated pump, a hydraulic motor, means providing a main-circuit including said second valve, motor, and power-operated pump, an operative connection between said spools, yieldable means acting on said spools to urge them toward closed position, one of said housings being provided with chambers respectively defined in part by end faces of the associated spool, said chambers communicating with the control circuit at points on opposite sides of said first valve whereby a pressure-differential at opposite sides of the first valve Will be applied through the chambers to the valve spool to urge both spools toward open positions, and means for automatically regulating the pressure-drop across the second valve.

16. A hydraulic system as set forth in claim 15 with the addition that said connection is adjustable to vary the axial position of one spool relative to that of the other.

17. A hydraulic system as set forth in claim 15 with the addition that said connection is a movement-multiplying connection.

18. A hydraulic system as set forth in claim 16 with the addition that the housing of one of said valves is provided interiorly with three axially spaced annular grooves the end ones of which communicate with the circuit in which such valve is incorporated, the spool of such valve having two axially spaced exterior annular grooves which, in the open condition of the valve connect the intermediate housing groove with the end housing grooves, respectively.

19. A hydraulic system as set forth in claim 16 with the addition that the housing of one of said valves is provided interiorly with four axially spaced annular grooves, the first and third of said grooves being connected into the associated circuit to jointly receive fluid therefrom and the second and fourth of said grooves being connected into the associated circuit to deliver fluid thereto, the spool of such valve having two axially spaced exterior annular grooves which, in the open condition of the valve respectively connect the first housing groove with the second and the third housing groove with the fourth.

References Cited by the Examiner UNITED STATES PATENTS 2,917,125 12/1959 Donner et al. ---52 X 2,974,491 3/1961 Cassaday et al. 6052 2,995,012 8/1961 Cassaday et al. 6052 3,016,708 1/1962 Gordon et al. 6052 SAMUEL LEVINE, Primary Examiner.

EDGAR W. GEOGHEGAN, Examiner. 

1. A HYDRAULIC SYSTEM, COMPRISING A CONTROL FLUID CIRCUIT INCLUDING A CONTROL PUMP, A MAIN-CIRCUIT INCLUDING A POWER-OPERATED PUMP AND A HYDRAULIC MOTOR, A FIRST ADJUSTABLE ORIFICE-FORMING MEANS ASSOCIATED WITH SAID CONTROL CIRCUIT FOR PROVIDING AN ORIFICE THROUGH WHICH FLOWS FLUID DISPLACED BY SAID CONTROL PUMP, A SECOND ADJUSTABLE ORIFICE-FORMING MEANS ASSOCIATED WITH SAID MAIN CIRCUIT PROVIDING AN ORIFICE THROUGH WHICH FLOWS FLUID DISPLACED IN OPERATION OF SAID MOTOR, MECHANISM RESPONSIVE TO THE RATE OF FLUID FLOW IN SAID CONTROL CIRCUIT FOR CONTROLLING BOTH SAID ORIFICE-FORMING MEANS TO VARY THE AREAS OF THE ORIFICES IN THE SAME SENSE AS SUCH FLOW-RATE VARIES, AND PRESSURE-REGULATING MEANS IN SAID MAIN CIRCUIT FOR AUTOMATICALLY REGULATING THE PRESSURE DROP ACROSS THE ORIFICE IN THAT CIRCUIT. 